Dynamics-based classification of semienclosed basins

This study is guided by other classifications of estuaries that are based on specified physical constraints. Previous classifications have been relevant and revealing of diagnostic functioning of estuaries in terms of their stratification and their baroclinically induced residual circulation. Those classifications, however, have disregarded residual circulations driven by tides, not only in estuaries, but in semienclosed basins in general. The classification proposed here uses tidally averaged, or residual, dynamics. Residual dynamics are explored via drivers, or forcing agents, of residual flow. Drivers are tides or density gradients and are counteracted by 'modifiers,' i.e., Earth's rotation or friction. The relative influence between forcing agents is characterized by a scaled non-dimensional densimetric tidal Froude number, Fr-0(2), which hinges on tidal current amplitude, water depth and reduced gravity. The competition between modifiers is determined by a non-dimensional Ekman number, Ek, which depends on the kinematic eddy viscosity, the latitude and the depth of the basin. The classification suggested identifies all types of estuaries, whose dynamics is influenced by density gradients, but also recognizes frictionless tidal basins, tidal rivers, coastal lagoons, and basins that switch their dynamics from being baroclinically forced to being tidally forced. This classification can also be interpreted as an assessment of the strength of mixing (in Ek) versus the strength of baroclinicity (in Fr-0(2)), which determines the residual flows in semienclosed basins. Additionally, the classification incorporates basins forced by wind stress and/or baroclinicity, and basins forced by wind stress and/or tides. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

This study is based on previous classifications of estuaries but focuses on residual dynamics driven by tides and density gradients. It introduces the concepts of drivers (tides or density gradients) and modifiers (Earth's rotation or friction) to characterize the relative influence between forcing agents. The classification identifies various types of estuaries influenced by density gradients and also includes basins forced by wind stress and/or baroclinicity or tides.